An imager, for example, a CMOS imager includes a focal plane array of pixel cells; each cell includes a photosensor, for example, a photogate, photoconductor or a photodiode overlying a substrate for producing a photo-generated charge in a doped region of the substrate. A readout circuit is provided for each pixel cell and typically includes at least a source follower transistor and a row select transistor for coupling the source follower transistor to a column output line. The pixel cell also typically has a charge storage node, for example, a floating diffusion node which is, in turn, connected to the gate of the source follower transistor. Charge generated by the photosensor is stored at the storage node. In some arrangements, the imager may also include a transistor for transferring charge from the photosensor to the storage node. The imager also typically includes a transistor to reset the storage node before it receives photo-generated charges.
In a CMOS imager pixel cell, for example, a four transistor (4T) pixel cell 100 as depicted in FIG. 1, the active elements of a pixel cell perform the functions of (1) photon to charge conversion by photodiode 102; (2) transfer of charge to the floating diffusion node 108 by the transfer transistor 104; (3) resetting the floating diffusion node to a known state before the transfer of charge to it by reset transistor 106; (4) selection of a pixel cell for readout by row select transistor 112; and (5) output and amplification of a signal representing a reset voltage and a pixel signal voltage based on the photo converted charges by source follower transistor 110, which has its gate connected to the floating diffusion node 108. The pixel of FIG. 1 is formed on a semiconductor substrate as part of an imager device pixel array.
FIG. 2 illustrates a block diagram of a CMOS imager device 908 having a pixel array 200 with each pixel cell being constructed as described above, or as other known pixel cell circuits. Pixel array 200 comprises a plurality of pixels arranged in a predetermined number of columns and rows (not shown). The pixels of each row in array 200 are all turned on at the same time by a row selected line, and the pixels of each column are selectively output by respective column select lines. A plurality of rows and column lines are provided for the entire array 200. The row lines are selectively activated in sequence by the row driver 210 in response to row address decoder 220 and the column select lines are selectively activated in sequence for each row activation by the column driver 260 in response to column address decoder 270. Thus, a row and column address is provided for each pixel.
The CMOS imager is operated by control circuit 250, which controls address decoders 220, 270 for selecting the appropriate row and column lines for pixel readout, and row and column driver circuitry 210, 260 which apply driving voltage to the drive transistors of the selected row and column lines. The pixel output signals typically include a pixel reset signal Vrst taken off of the floating diffusion node 108 when it is reset by reset transistor 106 and a pixel image signal Vsig, which is taken off the floating diffusion node 108 after photo-generated charges generated by an image are transferred to it. The Vrst and Vsig signals are read by a sample and hold circuit 265 and are subtracted by a differential amplifier 267, which produces a signal Vrst−Vsig for each pixel, which represents the amount of light impinging on the pixels. This difference signal is digitized by an analog to digital converter 275. The digitized pixel signals are then fed to an image processor 280 to form a digital image. The digitizing and image processing can be located on or off the imager chip. In some arrangements the differential signal Vrst−Vsig can be amplified as a differential signal and directly digitized by a differential analog to digital converter.
As shown in FIG. 1, the conventional four transistor (4T) pixel requires an operating voltage Vcc, as well as transfer TG, row select ROW and reset RST control signals for operation.